Microwave heating apparatus



' March 10, 1970 K. HILTON 3,500,012

MICROWAVE HEATING APPARATUS Filed March 6, 1968 2 Sheets-Sheet 1 l r j T I4 FIGI 78/ /5 March 10, 1970 HILTON MICROWAVE HEATING APPARATUS 2' Sheets-Sheet 2 Filed March 6, 1968 United States Patent US. Cl. 21910.61 16 Claims ABSTRACT OF THE DISCLOSURE A serpentine waveguide of which the waveguide sections are slotted for passage therethrough of sheet material to be heated and of which one end is provided with a microwave source, the other end is provided with a second microwave source whereby the field strength is made more nearly equal along the serpentine path. Preferably, means is provided for monitoring the power supplied by and incident on each source and for shutting both sources down if the power incident on either source becomes too great compared with the power supplied by it. A directionally-selective microwave-sensitive device suitable for incorporation in such monitoring means is provided, in which a resistor associated with a pick-up loop thereof is effectively shielded from the microwave field to be sensed by the loop.

BACKGROUND OF THE INVENTION Microwave heating apparatus of this kind is known, for example in specification No. 1,050,493 which discloses and claims such apparatus in which adjacent waveguide sections are connected to one another through irises, rather than by waveguide bends, whereby there is substantially no spacing between adjacent waveguide sections and correspondingly no cooling of the sheet material as it passes from one section to the next.

As described in the said specification, an assembly of wave-guide sections as described above has connected at one end a source of microwave energy and at the other end a dummy load. With a sufficient total path length for microwaves, i.e. the combined length of all the waveguide sections end-to-end, most of the microwave energy supplied by generator is absorbed by thesheet material, and the dummy load is required to absorb only the residual energy, perhaps or of the total. Since (apart from any effect which heating may have on the absorptive properties of the sheet material) the field strength of the microwave radiation falls off exponentially along the length of the serpentine microwave path, those waveguide sections which are nearer dummy load carry microwave energy at relatively low density and contribute relatively little to the heating of the sheet material.

SUMMARY OF THE INVENTION It is an object of the present invention to provide microwave heating apparatus, including a serpentine assembly of slotted waveguide sections, in which the various sections contribute more nearly equally to the heating effect than in the known apparatus referred to above. In

this specification, references to a serpentine assembly of waveguide sections are intended to include both assemblies in which adjacent sections are coupled to one another by waveguide bends and those in which the coupling is by means of irises.

According to the present invention, a serpentine assembly of slotted waveguides is provided at each end with a respective microwave generator and, preferably, with means for checking that the microwave power incident Patented Mar. 10, 1970 on each generator from the respective other generator is not excessive.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 shows a serpentine waveguide assembly fitted with two microwave generators and monitoring means,

FIGURE 2 shows, on a larger scale and in section, part of the monitoring means shown in FIGURE 1,

FIGURE 3 shows control circuitry incorporating said monitoring means and FIGURE 4 shows modified control circuitry.

The microwave heating apparatus shown in FIGURE 1 comprises an assembly 11 of iris-coupled slotted waveguide sections 12 arranged in serpentine manner for heating a sheet of material 13 fed through the slots thereof in the direction of the arrow 14 (or in the opposite direction). The assembly 11 is substantially in accordance with the disclosure of specification No. 1,050,493.

Two magnetrons 15 and 16 are connected to opposite ends of the assembly 11 by waveguide coupling sections 17 and 18 respectively, and it will be seen that microwave energy supplied to one end of the assembly 11 by a respective one of the magnetrons will be transmitted along the serpentine path provided thereby and incident on the other magnetron to the extent that it is not absorbed and attenuated by the sheet 13 before arrival there.

If it can be assumed for a moment that the magnetron 16 and its coupling section 18 is replaced by a dummy load which in known manner absorbs such residual power from the magnetron 15 as is not absorbed, during its transmission along the serpentine path provided by the assembly 11, by the material, it will be appreciated that for eflicient heating the length of the path provided by the assembly 11 must be sufficient, having regard to the properties of the material 13, that most of the power is absorbed by the material 13 and not by the dummy load. Since the absorption by the material 13 is substantially exponentially related to distance (along the serpentine path) from the magnetron 15, it will be understood that those of the waveguide sections 12 which are near the end of the assembly 11 to which the magnetron 15 is connected contribute relatively greatly, and those sections 12 which are near the other end of the assembly contribute relatively little, to the heating of the material 13 as it passes through the assembly 11. However, if the absorption by the material 13 is high (say greater than or the remaining 10% or 5% will be low enough to be tolerated by the second magnetron 16 connected instead of the assumed dummy load and itself supplying power at substantially the same rate as the magnetron 16. By this means the microwave field strength in the waveguide sections 12 nearest the magnetron 16 is increased to correspond to that in the sections 12 nearest the magnetron 15, and the heating effect made substantially more nearly uniform along the length of the assembly 11.

Magnetron manufacturers normally specify the maximum permissible ratio between the power reflected on to a magnetron by a load which it supplies and the power supplied by the magnetron to the load; and it will be understood that in the arrangement shown in FIGURE 1 the sheet of material 13 must attenuate the power supplied by each magnetron sufliciently that the total power incident at each magnetron from the assembly 11 (this total being principally energy supplied by the respective other magnetron) is in a sufiiciently small ratio with the power emitted by that magnetron. If this requirement were not met (for example, if the sheet 13 were completely withdrawn from the assembly 11), the magnetrons would rapidly be damaged.

To avoid such damage to the magnetrons, monitoring means are provided, associated with each magnetron, to check that the ratio of power incident on it to power supplied by it is within acceptable limits. As shown in FIGURE 1, the monitoring means comprise four sensing elements 19, 20, 21 and 22 of which one, say the element 19, is shown in greater detail and on a larger scale in FIGURE 2.

The sensing element 19 shown in FIGURE 2 is a resistive loop directional coupler operating on principles disclosed in a paper entitled The Reflectometer by H. R. Allen and C. D. Curling published at p. 25 of the Proceedings of the Institute of Electrical Engineers, 1949, 96, Part III. The coupling sections 17 and 18 are of nonsquare rectangular section, and the sensing elements are mounted in the wider walls thereof, one such wider wall being indicated as 17 in FIGURE 2. To accommodate and locate the sensing element 19, the wall 17 is formed with an aperture in which is secured a holder 23 provided with a locking screw 24. The sensing element comprises a body 25 which is inserted into the holder 23 and locked in position by means of the screw 24. The body 25 is formed with two bores 26 and 27. The bore 26 accommodates a resistor 28 of which one end is electrically earthed to the body 25 by a connection 29 and from the other end of which extends a conductor formed as a loop 30 projecting into the interior of the waveguide section and extending in. the longitudinal direction thereof. At its other end the loop 30 extends coaxially into the bore 27 and into the inner contact of a coaxial socket 31 screwed into the bore 27. The device shown in FIGURE 2 is coupled both electrically and magnetically to microwaves propagated along the waveguide 17 in the conventional H mode, the effect of the magnetic coupling being additive to that of the electrical coupling for microwaves propagated in the direction of the arrow 32 and mutually substractive for propagation in the reverse direction. By suitable choice of circuit constants, notatably the extent to which the loop 30 projects into the waveguide interior, it is arranged that the electrical and magnetic couplings are numerically equal so that the resultant microwavefrequency output at the socket 31, which is proportional to the field strength of microwaves propagated in the direction 32, is zero for microwaves propagated in the reverse direction. The dimensions of the part of the loop 30 which extends into the interior of the waveguide are much smaller than the microwave wavelength being monitored. The resistor 28 is preferably a carbon composition resistor, since such resistors have an R.F. resistance which is fairly constant and independent of their D.C. resistance in a range of approximately 100 ohms to one megohm.

The provision of the bore 26, and location of the resistor 28 wholly within it, are effective to shield the resistor substantially completely from the microwave field with which the loop 30 is coupled within the waveguide 17; and this shielding of the resistor enables the sensing element 19 to sense substantially greater field strengths than it could tolerate if the resistor were not so shielded.

The sensing element 20 is identical with the element 19, but is mounted with its body 25 rotated through 180. Thus the sensing element 19 provides a microwave-fre quency output which is proportional to the microwave field generated by the magnetron 15, and the sensing device 20 provides a microwave-frequency output which is in the same proportion to the microwave field incident on that magnetron. Similarly, the sensing elements 21 and 22 give outputs proportional respectively to the. fields incident on and generated by the magnetron 16.

As shown in FIGURE 3, the outputs from the sensing elements 19 and 20 are applied via respective R.F. detectors 33 and 34 to a comparator and amplifier circuit 35, the output of which is proportional to the ratio between the rectified inputs from the elements 19 and 20 and is applied to a relay 36 which, if the said ratio rises above the magnetron manufacturers specification for the magnetrons, actuates a contact 36 in a control circuit which interrupts the E.H.T. supply to both magnetrons and keeps it interrupted until reset. The rectified outputs from the sensing units 19 and 20 may also, as shown, be applied to microammeters 37 and 38 which then give indications of the emitted and incident power respectively. The sensing elements 21 and 22 are similarly connected to interrupt the magnetron E.H.T. supply; thus if either magnetron becomes subject to an excessive ratio of incident to emitted power, both are shut down until the matter has been investigated.

If desired, the RP. detectors 33 and 34 may be incorporated in the couplers 19 and 20, and 21 and 22, being located in each case in the bore 27 and connected electrically in series between the inner content of the socket 31 and the end of the loop 30 which extends into the bore 27.

As an alternative to using diode R.F. detectors, the R.F. output of a coupler such as the coupler 19 may be applied to a lamp of which the brilliance of illumination is proportional to the applied R.F. output, the lamp being arranged to illuminate a photosensitive cell whose output then also gives a measure of the RF. output. In such case, the lamp may be fitted in the bore 27, with its terminals connected to the body 25 and to the loop 30 respectively.

If desired, the sensing devices 19 and-22 may be omitted, and the signals proportional to the emitted powers of the two magnetrons may be obtained by alternative means. This possibility is represented in FIGURE 4, which shows (as in FIGURE 3), the sensing unit 20 connected via the rectifier 34 to supply the microammeter 38 and one input for the comparator and amplifier circuit 35 which is connected to the relay 36 which controls the contact 36. The other input for the circuit 35 is derived from the anode circuit of the magnetron 15, there being included in this circuit a resistor 39 across which, accordingly, there is developed a voltage which is proportional to the magnetron anode current. The anode current of the magnetron is, in fact, closely proportional to the power emitted by the magnetron, and it is thus satisfactory to employ, as shown, the voltage across the resistor 39 as the second input to the circuit 35. The microammeter 38 normally give an indication of the power incident on the magnetron 15, but indicates the power emitted by the magnetron 15 if the sensing unit 20 is turned through To permit the emitted power to be read in this way, a switch 40 is inserted between the rectifier 34 and the circuit 35. It will be understood that this switch would be opened before rotating the sensing element 20 to read emitted power. It will also be understood that the sensing unit 22 may similarly be omitted and the corresponding signal derived from the anode circuit of the magnetron 16.

What I claim is:

1. Microwave heating apparatus comprising a serpentine assembly of slotted Waveguide sections, providing a serpentine microwave path having two ends, a microwave energy source connected to supply microwave energy to the assembly at one of the said ends, wherein there is further provided a second microwave energy source connected to supply microwave energy to the assembly at the other of the said ends, and means for monitoring and comparing the microwave power supplied by an incident on each of said sources and switching off the power supplies to said sources on occurrence, in respect of either source, of a ratio of incident to supplied power which exceeds a predetermined value,

said means including, at each end of the said path, a

respective directionally-selective device arranged to respond to microwave power being supplied to said path at that end thereof.

2. Microwave heating apparatus comprising a serpena tine assembly of slotted Waveguide sections, providing a serpentine microwave path having two ends, a microwave energy source connected to supply microwave energy to the assembly at one of said ends, wherein there is further provided a second microwave energy source connected to supply microwave energy to the assembly at the other of the said ends, and means for monitoring and comparing the microwave power supplied by and incident on each of said sources and switching off the power supplies to said sources on occurrence, in respect of either source, of a ratio of incident to supplied power which exceeds a predetermined value, said means including means for sensing the electrical power consumption of each said source and thereby sensing indirectly the microwave power supplied thereby.

3. Microwave heating apparatus comprising a serpentine assembly of slotted waveguide sections, providing a serpentine microwave path having two ends, a microwave energy source connected to supply microwave energy to the assembly at one of the said ends, wherein there is further provided a second microwave energy source connected to supply microwave energy to the assembly at the other of said ends, and means for monitoring and comparing the microwave power supplied by and incident on each of said sources and switching off the power supplies to said sources on occurrence, in respect of either source, of a ratio of incident to supplied power which exceeds a predeterminned value, said means including, at each. end of said path, a respective directionally-selective sensitive device arranged to respond to microwave power being transmitted along the said path to that end thereof.

4. Microwave heating apparatus as claimed in claim 3 wherein the said means includes means for sensing the electrical power consumption of each said source and thereby sensing indirectly the microwave power supplied thereby.

5. Microwave heating apparatus as claimed in claim 3, wherein the said means includes, at each end of the said path, a respective directionally-selective device arranged to respond to microwave power being supplied to said path at that end thereof.

6. Microwave heating apparatus according to claim 3,-

wherein said directionally-selective microwave-sensitive device comprises an electrically conductive body having two ends and formed with a bore extending through the body from one end thereof to the other end thereof, a two-terminal connector mounted in one end of the bore with one terminal electrically connected to the body and another terminal electrically insulated therefrom, a wire loop projecting beyond'the other end of the said bore and having one end connected within the said bore to the said other terminal, and a resistor connecting the other end of the wire loop electrically to the body, wherein the body is formed with a second bore and the said resistor is disposed wholly within the second bore.

7. Microwave heating apparatus according to claim 5, wherein said directionally selective microwave-sensitive device comprises an electrically conductive body having two ends and formed with a bore extending through the body from one end thereof to the other end thereof, a two-terminal connector mounted in one end of the bore with one terminal electrically connected to the body and another terminal electrically insulated therefrom, a wire loop projecting beyond the other end of the said bore and having one end connected within the said bore to the said other terminal, and a resistor connecting the other end of the wire loop electrically to the body, wherein the body is formed with a second bore and the said resistor is disposed wholly within the second bore.

8. Microwave heating apparatus for strip material comprising:

(a) a waveguide formed with a slot for strip material to be heated and with a serpentine path for microwaves, said waveguide comprising (i) first electrically conductive wall means (ii) second electrically conductive wall means facing said first wall means (iii) a plurality of electrically conductive separators disposed parallel to and spaced from one another between said first and second wall means to define a plurality of parallel side by side legs of said serpentine path,

(iv) means disposed at opposite ends of said separators to transmit microwave energy from each of said legs to the next leg;

(b) a first microwave generator connected to said waveguide to supply microwave energy to one of said legs, said one leg being at one end of said waveguide such that said microwave energy supplied by said first generator passes through said serpentine path in one direction; and

(c) a second microwave generator connected to said wageguide to supply microwave energy to another of said legs, said other leg being at the opposite end of said waveguide to said one leg such that microwave energy from said second generator passes along said serpentine path in a direction opposite to said one direction.

9. Microwave heating apparatus as claimed in claim 8 and provided with means for monitoring and comparing the microwave power supplied by and incident on each of said generators and switching off the power supplies to said generators on occurrence, in respect of either generator, of a ratio of incident to supplied power which exceeds a predetermined value.

10. Microwave heating apparatus as claimed in claim 9, wherein the said means includes, at each end of the said path, a respective directionally-selective device arranged to respond to microwave power being supplied to said path at that end thereof.

11. Microwave heating apparatus as claimed in claim 9, wherein the said means includes means for sensing the electrical power consumption of each said generator and thereby sensing indirectly the microwave power supplied thereby.

12. Microwave heating apparatus as claimed in claim 9, wherein the said means includes, at each end of the said path, a respective directionally-selective sensitive device arranged to respond to microwave power being trans mitted along the said path to that end thereof,

13. Microwave heating apparatus as claimed in claim 12, wherein the said means includes means for sensing the electrical power consumption of each said generator and thereby sensing indirectly the microwave power supplied thereby.

14. Microwave heating apparatus as claimed in claim 12, wherein the said means includes, at each end of the said path, a respective directionally-selective device arranged to respond to microwave power being supplied to said path at that end thereof.

15. Microwave heating apparatus as claimed in claim 10, wherein said directionally-selective microwave-sensitive device comprising an electrically conductive body having two ends and formed with a bore extending through the body from one end thereof to the other end thereof, a two-terminal connector mounted in one end of the bore with one terminal electrically connected to the body and another terminal electrically insulated therefrom, a wire loop projecting beyond the other end of the said bore and having one end connected with the said bore to the said other terminal, and a resistor connecting the other end of the wire loop electrically to the body, wherein the body is formed with a second bore and the said resistor is disposed wholly within the second bore.

16. Microwave heating apparatus according to claim 14, wherein said directionally-selective microwave-sensitive device comprises an electrically conductive body having two ends and formed with a bore extending through the body from one end thereof to the other end thereof,

7 8 a two-terminal connector mounted in one end of the bore 3,227,971 1/1966 Stewart 333-17 with one terminal electrically connected to the body and 3,235,702 2/1966 Timmermans et al. 21910.55 another terminal electrically insulated therefrom, a Wire 3,304,399 2/1967 Timmermans et a1. loop projecting beyond the other end of the said bore 219-1061 X and having one end connected within the said bore to the 5 3,307,010 2/ 19-67 Piischner 219-10.61 X said other terminal, and a resistor connecting the other end of the wire loop electrically to the body, wherein the FOREIGN PATENTS body is formed with a second bore and the said resistor 1,050,493 12/1966 Great Britain.

is disposed wholly Within the second bore.

10 JOSEPH V. TRUHE, Primary Examiner References Cited L. H. BENDER, Assistant Examiner UNITED STATES PATENTS 2,498,719 2/1950 Spencer 333 17 2,575,799 11/1951 Doherty et al. s33 10 15 10,55;333 17 3,069,624 12/1962 Friedman 333-47 

